The so-called optical waveguides (also referred to as optical wirings and optical transmission paths) that transmit digital optical signals, rather than copper wirings which become very costly when accurate transmission of information is realized, have attracted much attention as a medium for short-distance ultrahigh-speed transmission inside the housings of electronic devices and apparatuses, such medium being capable to handle the explosive increase in the volume of transmitted information.
The optical waveguide, as referred to herein, is a structure in which a cladding material that is transparent at the wavelength of the light used and has a relatively low refractive index encloses the periphery of a linear transmission path formed from a core material with a relatively high refractive index, or is arranged above and below a planar transmission path. An optical fiber is an optical waveguide, but because the mounting density of a core in an optical fiber is difficult to increase, in terms of realizing a high density and ultrahigh-speed transmission at the same time, the most promising is a resin optical waveguide in which a plurality of linear cores or planar cores is formed inside a cladding layer by patterning performed by exposure of a plane. An optical waveguide having linear cores is called a ridge optical waveguide or a channel optical waveguide, and an optical waveguide having planar cores is called a slab optical waveguide or a planar optical waveguide.
Methods including spin coating or die coating of a resin which is liquid at room temperature on a base material and curing are known as means for realizing a resin optical waveguide by optical exposure, and since such methods are the simplest to implement at a laboratory level, a large number thereof have been developed, but in industrial implementation of such methods, problems are associated with the restriction placed on the size of the work and the reduction of thickness unevenness. Accordingly, optical wiring materials of the so-called dry film type are optimum for industrial production, and various types thereof have been developed.
A material for an optical waveguide of a dry film type is a product obtained by arranging at least an uncured resin for an optical waveguide which is solid at room temperature on a carrier base material (also referred to as carrier film, base film, and support film), and the processing thereof involves laminating the resin surface of the dry film for an optical waveguide on some planar body and then curing and patterning.
A protective film (also referred to as cover film, separator, or masking film) is often arranged on the surface of the resin for an optical waveguide on the side which is not in contact with the carrier base material with the object of protecting the resin for an optical waveguide. Where the surface of the resin for an optical waveguide is laminated on a certain planar body, the protective film needs to be peeled off and removed. Therefore, measures are usually taken to facilitate the peeling rather than to increase the adhesive force between the protective film and the resin for an optical waveguide. Further, in this case, where the protective film is peeled off, it is necessary that the peeling proceed at the interface of the resin for an optical waveguide and the protective film. Therefore, adhesion at this interface needs to be less than that at the interface of the carrier base material and the resin for an optical waveguide.
Several techniques for manufacturing an optical waveform from a resin material have been reported (see Patent Literatures 1 to 4). A dry film technique relating to a dry film for solder resist, coverlay, or etching resist has also been reported (Patent Literature 5).
However, Patent Literature 1 discloses a method for forming an optical waveguide by using a dry film of a structure which is constituted by a base film and a resin layer formed on the base film and in which a cover film of polyethylene or polypropylene is optionally laminated as a protective film on the opposite side of the base film, and the resin layer is sandwiched between the base film and the cover film. With respect to the cover film, only the material thereof is disclosed, and the roughness thereof is not described at all.
Patent Literature 2 discloses a manufacturing method for an optical waveguide in which a resin for forming a cladding layer which is formed on a base material is cured to form a lower cladding, a resin film for forming a core layer is laminated on the lower cladding layer to form a core layer, the core layer is exposed and developed to form a core pattern, and a resin for forming a cladding layer which is formed such as to embed the core pattern is cured to form an upper cladding layer. The resin for forming a core is specified to be in the form of a film, but the resin for forming a cladding may be also in the form of a film, and it is indicated that where the resin films for core and cladding are both used to form resin layers on a support film which is not to be eventually used for a base material of an optical waveguide, that is, when the support film needs to be peeled off and removed from the resin layer, it is preferred that the support film be not subjected to matting treatment such as corona treatment and sandblasting for increasing the adhesive force between the support film and the resin layer, or adhesive treatment such as coating of an easy-adhesion resin.
Patent Literature 3 discloses a method for manufacturing an optical-electrical composite substrate in which an electric wiring substrate provided with a lower cladding layer is obtained, and a core pattern and an upper cladding layer are successively formed on the lower cladding layer thereby configuring an optical waveguide. It is also indicated that a resin for forming a cladding layer and a resin for forming a core layer are preferably used in the form of a film, a resin layer is formed on a base material film serving as a support that supports each resin film, PET (polyethylene terephthalate), polypropylene, or polyethylene is preferably used as the base material film, and mold parting treatment and antistatic treatment may be implemented to facilitate subsequent separation of the resin layer. It is further disclosed that a protective film may be bonded to the resin films for the core and cladding with consideration for film protection and winding ability when a roll-shaped configuration is produced, that a protective film similar to that in the example of the base material film can be used, and that mold parting treatment and antistatic treatment may be optionally implemented.
Patent Document 4 discloses a method for manufacturing a flexible optical waveguide in which a first cladding layer is formed, a resin film for forming a core layer is laminated on at least one end portion on top of the first cladding layer to form a first core layer, a resin film for forming a core layer is laminated over the entire surface of the first core layer and the first cladding layer to form a second core layer, the first core layer and the second core layer are patterned to form a core pattern of an optical waveguide, and a second cladding layer is formed on the core pattern and the first cladding layer to embed the core pattern. It is also indicated that the base material of the resin film for forming a cladding layer, for example, may be subjected to physical or chemical surface treatment such as oxidation and roughening to improve adhesivity with the resin for forming a cladding layer, corona treatment, chromium oxide treatment, flame treatment, hot air treatment, ozone and UV treatment are presented as examples of oxidation methods, and the so-called bonding treatment such as sandblasting and solvent treatment are presented as examples of roughening. Since the base material film of the resin film for forming a cladding layer is eventually positioned for use on the outermost surface of the flexible optical waveguide, it is preferred that the abovementioned surface treatment be performed to obtain better adhesivity with the cladding resin. Meanwhile, although an example is also disclosed in which the base material film is peeled off and removed from at least one side to reduce the thickness of the flexible optical waveguide, or the base material film is peeled off from both sides to reduce buckling of the flexible optical waveguide, in the disclosed method, humidification is performed under high-temperature and high-humidity conditions with the object of easily peeling off the base material film and the adhesion between the base material film and the resin for cladding is reduced to facilitate peeling, on the basis of the aforementioned presumption that higher adhesion between the base material film and resin for cladding is preferred. A structure is also disclosed in which a protective film (separator or masking film) is laminated, with the object of protecting the resin film or improving winding ability at the time of manufacture, on the resin film for forming a cladding layer and the resin film for forming a core layer on the surface of the resin films on the side opposite that of the base material film, and it is indicated that the protective film is preferably not subjected to the aforementioned bonding treatment to facilitate the peeling of the resin for forming a cladding and the resin for forming a core. It is further disclosed that the so-called vacuum lamination in which heating and pressurization are performed under a reduced pressure is preferred from the standpoint of improving adhesion and adaptability when the resin film for forming a core is laminated, and that the lamination is preferably performed using a roll laminator to prevent the inclusion of air bubbles between the first cladding layer and the core layer.
Patent Literature 5 discloses a photosensitive film for laminating on a printed wiring board, the photosensitive film being characterized in that the surface roughness of a protective film is equal to or greater than 0.5 μm as an arithmetic average roughness (Ra) in a measurement range with a cut-off value of 0.08 mm to 8 mm and an evaluation length of 0.4 mm to 40 mm, the photosensitive composition layer has flowability such that where a static load of 0.25 kg/mm2 is applied to the photosensitive composition layer with a layer thickness of 2 mm at a temperature of 30° C., the film thickness change amount in a time period from after 10 sec to after 600 sec since the application of the load is within a range of 50 μm to 800 μm, the protective film imparts surface roughness to the photosensitive composition layer, and the surface roughness is maintained prior to the lamination on the printed wiring board and eliminated by the pressurization during the lamination. As described in paragraph [0002] of Patent Literature 5, with the so-called solder resist, a coverlay of a flexible printed wiring board, and an etching resist which is used in the formation of copper circuits of printed wiring boards, the resin needs to be fluidized to a high level in order to embed a conductive pattern in the resin, without air bubbles, but since the tackiness of the resin is thereby increased, protruding conductors cannot be sufficiently covered and the protective function of the film cannot be realized, or air bubbles remain inside the surface defects of the stretched copper laminated boards. The photosensitive film with the above-described properties serves to resolve such problems.